An electroluminescent diode sensor (EDS) can be used as a sensor. When energy is absorbed by a π-conjugated polymer, electrons are promoted from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) creating an excited state. The excited electron can relax down to the HOMO releasing a photon which is called a radiative process. Or the electron can relax down to the HOMO by a process that does not emit light, a non-radiative process.